This invention relates to tuneable microcavities, and especially their use in luminescent devices.
FIG. 1 shows a planar microcavity. This is a Fabry-Perot resonator with two mirrors 1,2 spaced apart by a cavity 3 which contains a photon-emitting material 4. The mirror separation is of the order of the optical wavelength, so that the resonant frequency of the cavity corresponds to an optical frequency. Such a structure therefore has a narrow emission spectrum, allowing emission only at the resonance wavelength(s) of the cavity. It is also capable of enhancing the emission at a certain wavelength compared to the free-space emission of the luminescent material (see J. Gruner et al. J Appl. Phys. 80, 207 (1996)). These properties have proved to be useful for light-emitting devices based on broad bandwidth emitters such as organic molecular or polymeric materials, providing the improved colour purity and spectral tuneability required for multi-colour display applications (see U. Lemmer et al. Appl. Phys. Lett. 66, 1301 (1996); H. F. Wittmann et al. Adv. Mater. 6, 541 (1995); and A. Dodabalapur et al. Electronics Letters 30, 1000 (1994)).
One type of electroluminescent device is described in PCT/WO90/13148, the contents of which are incorporated herein by reference. The basic structure of this device is a light-emitting polymer film (for instance a film of a poly(p-phenylenevinylene)--"PPV") sandwiched between two electrodes, one of which injects electrons and the other of which injects holes. It is believed that the electrons and holes excite the polymer film, emitting photons. These devices have potential as flat panel displays.
In more detail, such an organic electroluminescent device ("OLED") typically comprises an anode for injecting the positive charge carriers, a cathode for injecting the negative charge carriers and, sandwiched between the electrodes, at least one organoluminescent layer. The anode is typically a layer of indium-tin oxide ("ITO") which is deposited on a glass substrate. The organic layer(s) are then deposited on the anode and the cathode is then deposited on the organic layer(s) by, for example, evaporating or sputtering. The device is then packaged for protection.
Organic light emitting devices have been incorporated in microcavity structures (see the papers by Lemmer et al. and Wittmann et al.). It has been shown that by using different thicknesses of a patterned inert filler it is possible to fabricate microcavity light-emitting devices with emission peaks from 490 nm to 630 nm from a single organic semiconductor (see the paper by Dodabalapur et al.). However, in such devices the resonance wavelength, and hence the emission colour, is fixed when the device is fabricated. Therefore, the colour (for instance of an individual pixel in a multi-pixel display) cannot be varied independently and/or externally after fabrication.